# Difference between revisions of "Optically trapped aqueous droplets for single molecule studies"

Zach Wissner-Gross (May 2, 2009)

## Information

Optically trapped aqueous droplets for single molecule studies

J. E. Reiner, A. M. Crawford, R. B. Kishore, Lori S. Goldner, K. Helmerson

Applied Physics Letters, 2006, 89, 013904

## Soft matter keywords

Emulsion, surface tension, optical trapping

## Summary

Figure 1: Photobleaching of one (left), two (middle), and three (right) fluorescent molecules in a single aqueous droplet.

To optically trap microparticles, one requirement is that the particle have a higher index of refraction than its surrounding medium. While small plastics beads are easy to trap, water droplets are more difficult to to water's relatively low refractive index of 1.33. Reiner et al. overcome this difficulty by mixing water with a low-index (1.29) hydrophobic fluorocarbon medium, followed by ultrasonication to produce droplets approximately 2 $\mu$m in diameter.

This paper is constructed around several applications of the optical trapping system. First, the authors use it to detect single molecules by trapping a droplet containing a handful of fluorescent molecules, and then photobleaching them, a stochastic process that reveals integer multiples of fluorescence (Figure 1). The authors also demonstrate fluorescence resonance energy transfer with a single molecule, contained, of course, in an aqueous droplet. Finally, they fuse together two droplets by bringing them together with two different laser beams (Figure 2).

## Soft matter discussion

In this system, there are really two types of forces at work: there are optical forces, but then there are also forces created by the surface tension of the aqueous droplets. Optical traps generate forces that are typically on the order of piconewtons, or $10^{-12}$ N. What about the capillary forces?

Well, these should be on the scale of the surface tension (about 70 mN/m) multiplied by the size of the drops (again, about 2 $\mu$m), or roughly 140 nN. So in this example, the capillary forces are much, much greater than the optically-generated forces, explaining why the droplets do not appear deformed (but are rather quite spherical) as they are manipulated by the optical traps (see Figure 2).

Colin Bain (who gave a seminar after class earlier this semester) and researchers at Durham University in England found that by adding a particular surfactant to heptane droplets they were able to sufficiently reduce the surface tension of the droplets so that they were much less than optical forces [1]. This allowed them to shape droplets with lasers, and even to pull them apart, thereby creating nanofluidic channels between daughter droplets.